Tunable filters can consist of Bragg gratings photo-induced onto portions of waveguides such as optical fibers or planar waveguides. This kind of filter constitutes a component integrated into an optical guide and considerably reduces insertion losses and polarization-dependent losses.
An optical fiber conventionally comprises an optical core, whose function is to transmit and possibly to amplify an optical signal, surrounded by optical cladding, whose function is to confine the optical signal in the core. To this end, the refractive index ncore of the core is greater than the refractive index ncladding of the cladding. Thus the core and the cladding form a waveguide. The cladding is itself surrounded by an external medium whose refractive index next is greater than or less than ncladding. The core-cladding combination associated with the external medium again forms a waveguide.
To write the Bragg grating, the core and/or the cladding of the fiber can be doped to render it or them photosensitive, for example with germanium (Ge).
A Bragg grating conventionally consists of a local periodic variation in the refractive index of the guide. The pitch of the grating, i.e. the distance between two index variations in the guide, can be constant, varying or multiple. In the case of a grating with a constant pitch Λ, the periodic variation of the refractive index distributed along the optical transmission axis in a portion of the guide creates a mirror which is reflective for a given wavelength known as the Bragg wavelength (the filtered wavelength), in accordance with the following equation, in which neff is the effective index of the fundamental mode in the core of the guide:λB=2.Λ.neff
One technique for varying the pitch of the grating, and thus the filtered wavelength, is to stretch the fiber.
The stretching can be mechanical and consist, for example, in pulling on each end of the fiber. This kind of stretching is complicated to carry out and to monitor.